1. Field of the Invention
The present invention relates to an image forming apparatus and particularly to a construction of a detecting device for detecting the presence or absence of a sheet.
2. Description of the Related Art
An image forming apparatus, which can form an image on both sides of a sheet, can be cited as a conventional example of an image forming apparatus such as a printer, facsimile, and copier. FIG. 7 is a view illustrating a construction of a laser beam printer, which is an example of such a conventional image forming apparatus.
In FIG. 7, a laser beam printer 101 comprises an image forming portion 102 for forming an image on a sheet P and a feeding unit 105 for feeding the sheet P to the image forming portion 102. Moreover, this laser beam printer 101 is provided with a sheet re-feeding and conveying portion 103 for feeding sheets to the image forming portion 102 again for forming an image on the back side of the sheet after an image is formed on one side of the sheet, and a discharge path 150 for discharging the sheet on which an image is formed on one side or both sides.
Here, the image forming portion 102 comprises a process cartridge 107 including a photosensitive drum 108, a transfer roller 109, and the feeding unit 105 is provided with a sheet feed cassette 104 on which the sheet P is stacked, and a feeding roller 106. The sheet re-feeding and conveying portion 103 comprises a switchback portion provided with a flapper 139, an after-fixation roller 117 also used as a switchback roller, and a switchback rotatable member 119. Moreover, the sheet re-feeding and conveying portion 103 comprises a reversing path 137 including a pair of conveying rollers (not shown) and a re-conveying path R on which a lateral registration correction portion 123 including an oblique-feed rotatable member 126, and a sheet re-feeding roller 122 are arranged.
A fixing unit 110 and a laser scanning unit 112 are also provided. A sheet discharge portion 113 includes a pair of switchback rollers 114, 115 and a sheet discharge/stacking table 116. A rotatable member 118 is brought into pressure contact with the after-fixation roller 117.
A sensor flag 150 constitutes a presence or absence sensor (not shown) for detecting the presence or absence of the sheet P inside the sheet feed cassette 104. This sensor flag 150 is brought into contact with the sheet P by the gravitational force of the sensor flag 150 or an urging unit (not shown). A controller board 111 controls the operation of the laser beam printer 101. The controller board 111 detects the presence or absence of the sheet P inside the sheet feed cassette 104 based on a signal from the presence or absence sensor.
In the above laser beam printer 101, when the controller board 111 issues a print signal based on image information sent from a personal computer (not shown), the sheet P stacked on the sheet feed cassette 104 is fed by the feeding roller 106 one by one. After that, the sheet P is conveyed to a nip between the photosensitive drum 108 and the transfer roller 109 and the fixing unit 110 in this order, and an image is formed on the sheet.
Here, if the controller board 111 has received a signal for forming an image on one side (one-side printing), then, the sheet P is held between the after-fixation roller 117 and the rotatable member 118 and fed directly through the discharge path 150 to the sheet discharge portion 113 by forward rotation of the after-fixation roller 117. Thereafter, the sheet P is discharged by the pair of switchback rollers 114, 115 onto the sheet discharge/stacking table 116.
Also, when the controller board 111 has received a signal for forming images on both sides (double-side printing), the pair of switchback rollers 114, 115 is reversely rotated, and the flapper 139 is switched. By this, the sheet P having been conveyed to the discharge path 150 is guided by the switched flapper 139 to the nip between the after-fixation roller 117 and the switchback rotatable member 119 and subsequently fed to the sheet re-feeding and conveying portion 103 with its leading edge and trailing edge reversed.
Then, the sheet P having been thus fed to the sheet re-feeding and conveying portion 103 in the state where the leading edge and trailing edge are reversed enters the re-conveying path R through the reversing path 137 and then, reaches the lateral registration correction portion 123. Here, at the lateral registration correction portion 123, an oblique-feed force is exerted on the sheet P by an oblique-feed roller 125 and the oblique-feed rotatable member 126 in contact with this oblique-feed roller 125, by which the sheet P is conveyed and brought closer to a reference plate 124 for correcting the skew feed of the sheet P.
When the sheet P reaches the reference plate 124 after that, the side end of the sheet P is conveyed along the reference plate 124, and thereby the sheet P is corrected to the proper state without skew feed or displacement. Then, after this lateral registration correction, the sheet P is fed to the image forming portion 102 again by this sheet re-feeding roller 122 and a rotatable member 130 in contact with this sheet re-feeding roller 122, and an image is formed on the second side (back face). After that, the sheet P is conveyed to the sheet discharge portion 113 and stacked onto the sheet discharge/stacking table 116.
In this type of laser beam printer, a jam might occur in the re-conveying path R, for example. And as disclosed in Japanese Patent Application Laid-open No. 2002-362811 and Japanese Patent Application Laid-Open No. 2002-012336, some of laser beam printers are designed such that in the case of a jam, in order to remove the jammed sheet, a lower conveying guide 127 constituting the bottom surface of the re-conveying path R is rotated clockwise, for example, so as to open the re-conveying path R.
This type of laser beam printer is designed such that, as shown in FIG. 7, for example, a jam handling lever 146 is provided at a laser beam printer body 101A, and when a jam occurs in the re-conveying path R, the jam handling lever 146 is pulled outward. Here, if the jam handling lever 146 is pulled in this way, a cam 141 provided at the jam handling lever 146 is rotated clockwise, and locking by the cam 141 on a rotating end of the lower conveying guide 127 is released.
As a result, as shown in FIG. 8, the lower conveying guide 127 is rotated downward around a fulcrum 127a as its center of rotation, and by this downward rotation of the lower conveying guide 127, the re-conveying path R is opened. Then, the jamming is cleared by pulling out the jammed sheet P in a Z direction, that is, to the front side of the laser beam printer.
This type of conventional laser beam printer (image forming apparatus) is designed such that, if the re-conveying path R is opened by rotating the lower conveying guide 127 downward, the sheet P is pulled to the front side of the laser beam printer in order to remove the sheet P as mentioned above.
However, when the sheet P is thus pulled out, the sheet P might be caught by the sensor flag 150, and if the sheet P is caught in this way, the sheet P is torn and might remain in the laser beam printer.